The reason for the shortage is a global manufacturing issue.
It is simply not being made at the moment (at all, or in previous quantities) and it is not clear when normal production will resume.
@scaevola do you have any further info/links on this? As you say, with all drugs there are shortages from time to time which can be due to varying factors, but I haven't seen anything that infers production of HRT has stopped totally by all manufacturers.
@ItsAllGoingToBeFine thanks for the link - useful to know.
As with any drugs, I'm sure any short term issues will be resolved and there will be enough to go around for everyone.